The present invention relates generally to a flame retardant multi-layered structure. More particularly, the present invention relates to a flame retardant microstructure bearing multi-layered structure.
The use of a microstructure bearing article as a brightness enhancement film is known in the art. Illustrative brightness enhancement films are described in Whitehead, U.S. Pat. No. 4,542,449; Lu et al., U.S. Pat. No. 5,175,030; and Lu, U.S. Pat. No. 5,183,597. The Lu et al. and Lu patents disclose microstructure bearing composite polymeric articles and a method of forming microstructure bearing composite polymeric articles. The Lu et al. and Lu patents disclose forming the microstructure with desired optical properties, such as total internal reflection.
Microstructure bearing articles are made in a variety of forms. One such form includes a series of alternating tips and grooves. One example of such a form is brightness enhancement film, which has a regular repeating pattern of symmetrical tips and grooves. Other examples include patterns in which the tips and grooves are not symmetrical and in which the size, orientation, or distance between the tips and grooves is not uniform. When the included angle of the grooves is in the range of about 70.degree. to about 120.degree. and especially within 80.degree. to about 100.degree. , the article with alternating tips and grooves is useful as a brightness enhancement film.
Brightness enhancement film is typically incorporated into a liquid crystal display, which is then incorporated into an electronic device such as a lap-top computer. Devices such as lap-top computers are required to have a flammability resistance rating. For making lap-top computers, manufacturers prefer to use components that have an established flammability resistance rating. If all the components in a lap-top computer have a satisfactory flammability resistance rating, the manufacturer of the lap-top computer does not have to submit the assembled lap-top computer for a flammability resistance rating.
Prior art brightness enhancement films include "Brightness Enhancement Film", a version made from polycarbonate thermoplastic, sold by Minnesota Mining and Manufacturing Company, St. Paul, Minn. and "DIAART" a version made of a radiation cured microstructured layer on a polyester base, sold by Mitsubishi Rayon, Tokyo, Japan.
Conventional prior art brightness enhancement films manufactured from polycarbonate and radiation-cured composites, however, do not exhibit sufficient flammability resistance characteristics to qualify for the lowest flammability rating from a recognized rating organization, such as Underwriters' Laboratories, Inc. There are several difficulties that must be overcome when constructing a brightness enhancement film with flammability resistance characteristics.
Prior art flame retardant chemicals that are used to reduce the flammability of polymers are typically insoluble salts, such as antimony oxide, or highly crystalline compounds, such as decabromobiphenyl. These compounds are not suitable for use with radiation curable resins, which are preferred for making brightness enhancement film. Such compounds inhibit radiation curing of the resin or reduce the desired optical qualities of the film.